The invention relates in general to communication protocols for implementing a connection between two data communication devices and in particular to the management of turns and of collision situations in the so-called half-duplex system based on alternating transmission and reception.
Data transmission systems based on infra-red radiation, i.e. infra-red links, have recently achieved popularity especially in applications where rapid transmission of data is required between two devices located relatively close to each other but where it is not desired to link said devices together by a cable connection. It is an object of the present invention to provide advantageously a protocol suitable for an infra-red link in particular and as an example embodiment the transmission of data between a digital communications adapter for connection to a computer and a mobile telephone for a digital cellular network is described. The invention is not, however, restricted to the example embodiment described but can be applied also in connection with other forms of data transmission and the data transmission call take place between any devices whatsoever that are equipped with suitable means of transmission and reception.
In all transmitter/receiver devices based on radiation and also in many devices using other forms of data communication it is often a problem that if the transmitter and receiver of the device are located in proximity to each other and operate on the same frequency band, part of the transmitted signal reaches the receiver and causes interference. The problem applies particularly to infra-red devices which are used in relatively small enclosed spaces where the infra-red signal, which is propagated with the velocity of light, is reflected from walls and other nearby surfaces and thereby reaches the receiver of the transmitting device practically simultaneous with the transmission. The same problem applies to devices based on fibre optics, in which reflections are caused by joints and discontinuities in the optical fibre. In fibre optics the easiest solution is to employ separate fibres for transmission and reception, but this involves doubled cable costs compared with single fibre equipment.
Another problem, the solution of which by some means is generally an essential part of a communication protocol, is the resolution of so-called collision situations.
Especially in situations in which regular transmission of data is not in progress but in which the devices at intervals send maintenance and/or control messages, for example to verify that the possibility of communication exists, or initialization messages for the commencement of regular data transmission, it may occur that two devices send messages on the common channel at the same time so that the messages become mixed up and neither of them can be understood. Such a situation is called a collision situation. A communication protocol must define how a communication connection is to be normalized after a collision.
From U.S. Pat. No. 4,289,373 (Sugimoto et al.) a solution of the first-mentioned problem is known using a bidirectional optical fibre link in which low-pass and high-pass filters are used to separate radiation in the transmit and receive directions on the basis of wavelength. The same principle can be applied in all data transmission systems based on electromagnetic radiation in which it is possible to separate just a small part of the frequency spectrum for use at one time. Generally the problem is that filters which possess sufficient selectivity to separate two frequency bands relatively close to each other are either bulky or costly or both. Further, the use of optical filters generally means that the transmission and reception frequency bands of a device are fixed, so that the device can only be used to communicate with devices of a certain second type in which the corresponding frequency bands are employed in the opposite order with respect to transmission and reception. The aforesaid patent publication concentrates on the implementation of the equipment and does not describe any protocol which could be used to implement the transmission of data.
From the patent publication WO 85/02271 (Kosman et al.) a bi-directional optical fibre link is known which is suitable for high-speed asynchronous data transmission between several terminal devices. With the equipment described simultaneous bi-directional communication is possible in an optical cable, but with the exception of certain ready signals the publication does not deal with protocol type arrangements. Moreover, the system is probably subject to the aforesaid reflection problems since it comprises a transmitter device and a receiver device located one after the other at one end of the same optical cable. In the publication no arrangements are described for avoiding problems caused by reflections. U.S. Pat. No. 4,399,563 (Greenberg) discloses a fibre optics link for generally known so-called half-duplex communication between two devices. By the half-duplex principle is meant an arrangement in which bi-directional communication between two transmitter/receiver devices is possible, but in which only one transmitter device and the corresponding receiver device at the other end are active at any one time. The receiver located in conjunction with the active transmitting device and the transmitter at the receiving end are switched off or their connection with the data transmission channel is otherwise broken so that they do not cause interference. An essential part of the protocol which governs the link is a procedure for sharing transmission and reception turns.
In the system described in U.S. Pat. No. 4,399,563 the division of turns is implemented by one participant in the link being a so-called master station and the other being a so-called slave station. The master station sends signals at regular intervals with the slave station being then switched to receive. The length of the signal block transmitted at one time is known in advance to both stations and after sending the signal block in question the master station disables its transmitter and enables its receiver. Almost simultaneously (later only by the duration of the physical delay of the transmission channel) the slave station finds it has received a signal block corresponding to the agreed length, whereon it disables its receiver and enables its transmitter to send a reply signal of the same length. The reply process must occur so rapidly that the master station has time to receive the reply of the slave station before the next occurrence of its own regularly recurring transmission turn.
Among the disadvantages of the system described are that the master and slave stations must operate synchronously, which requires synchronization of the clocks governing their operation. Further, the publication does not describe what actions are required for the initialization of communication. Because the link in question is a fixed fibre optics link, it has apparently been supposed in the publication that the master and slave designations are permanent and that only the master station can take action to commence communication.